Growth hormone-releasing hormone (GH-RH) is a peptide belonging to the secretin/glucagon family of neuroendocrine and gastrointestinal hormones, a family that also includes vasoactive intestinal peptide (VIP), pituitary adenylate cyclase activating peptide (PACAP) and others. Human GH-RH (hGH-RH) peptide is comprised of 44 amino acid residues. The best known site of production of GH-RH is the hypothalamus, but it was found that various peripheral organs also synthesize it. hGH-RH is also produced, sometimes in large quantities, by human malignant tissues (cancers) of diverse origin.
GH-RH exerts various physiological and pathophysiological functions. Hypothalamic GH-RH is an endocrine releasing hormone that, acting through specific GH-RH receptors on the pituitary, regulates the secretion of pituitary growth hormone (GH). The physiological functions of GH-RH in extrapituitary tissues are less clear. However, there is increasing evidence for the role of GH-RH as an autocrine/paracrine growth factor in various cancers. Splice variant (SV) receptors for GH-RH, different from those expressed in the pituitary, have been described in a wide range of human cancers and in some normal peripheral organs. The actions of tumoral autocrine/paracrine GH-RH could be exerted on these receptors. In addition, receptors for VIP and other, as yet unidentified receptors of this family, could all be targets of local GH-RH.
In view of the role of GH-RH as an endocrine regulator of GH release, novel therapeutic strategies, based on the use of agonistic and antagonistic analogs of GH-RH, have been devised for the treatment of various pathological conditions.
GH is a polypeptide having 191 amino acids that stimulates the production of different growth factors, e.g. insulin-like growth factor I (IGF-I), and consequently promotes growth of numerous tissues (skeleton, connective tissue, muscle and viscera) and stimulates various physiological activities (raising the synthesis of nucleic acids and proteins, and raising lipolysis, but lowering urea secretion). Release of pituitary GH is under the control of releasing and inhibiting factors secreted by the hypothalamus, the primary releasing factors being GH-RH and ghrelin, and the main inhibiting factor being somatostatin.
GH has been implicated in several diseases. One disease in which GH is involved is acromegaly, in which excessive levels of GH are present. The abnormally enlarged facial and extremity bones, and the cardiovascular symptoms of this disease can be treated by administering a GH-RH antagonist. Further diseases involving GH are diabetic retinopathy and diabetic nephropathy. The damage to the retina and kidneys respectively in these diseases, believed to be due to hypersecretion of GH, results in blindness or reduction in kidney function. This damage can be prevented or slowed by administration of an effective GH-RH antagonist.
In an effort to intervene in these disease and other conditions, some investigators have attempted to control GH and IGF-I levels by using analogs of somatostatin, an inhibitor of GH release. However, somatostatin analogs, if administered alone, do not suppress GH or IGF-I levels to a desired degree. If administered in combination with a GH-RH antagonist, somatostatin analogs will suppress IGF-I levels much better.
However, the main applications of GH-RH antagonists are in the field of cancer (reviewed in Schally A V and Varga J L, Trends Endocrinol Metab 10: 383-391, 1999; Schally A V et al, Frontiers Neuroendocrinol 22: 248-291, 2001; Schally A V and Comaru-Schally A M, in: Kufe D W, Pollock R E, Weichselbaum R R, Bast Jr. R C, Gansler T S, Holland J F, Frei III E, Eds. Cancer Medicine, 6th ed. Hamilton, Ontario: BC. Decker, Inc., 2003, p. 911-926). GH-RH antagonists inhibit the proliferation of malignancies by indirect endocrine mechanisms based on the inhibition of pituitary GH release and resulting in the decrease of serum levels of GH and IGF-I, as well as by direct effects on the tumor tissue.
GH-RH and its tumoral splice variant (SV) receptors are present in human cancers of the lung, prostate, breast, ovary, endometrium, stomach, intestine, pancreas, kidney, and bone (see Halmos G et al, Proc Natl Acad Sci USA 97: 10555-10560, 2000; Rekasi Z et al, Proc Natl Acad Sci USA 97: 10561-10566, 2000; Schally A V et al, Frontiers Neuroendocrinol 22: 248-291, 2001; Schally A V and Comaru-Schally A M, in: Kufe D W, Pollock R E, Weichselbaum R R, Bast Jr. R C, Gansler T S, Holland J F, Frei III E, Eds. Cancer Medicine, 6th ed. Hamilton, Ontario: BC. Decker, Inc., 2003, p. 911-926). Tumoral GH-RH has been shown or it is suspected to act as an autocrine growth factor in these malignancies. Antagonistic analogs of GH-RH can inhibit the stimulatory activity of GH-RH and exert direct antiproliferative effects in vitro on cancer cells, and in vivo on tumors. Direct antiproliferative effects of GH-RH antagonists are exerted on tumoral receptors (binding sites). In addition to the specific tumoral SV receptors for GH-RH, receptors for VIP and other, as yet unidentified receptors of this family, are targets of GH-RH antagonists.
In addition to endocrine inhibitory effects on serum GH and IGF-I, GH-RH antagonists have been found to reduce the autocrine and paracrine production of several tumor growth factors and/or downregulate their receptors. These growth factors include IGF-I, IGF-II, GH, vascular endothelial growth factor (VEGF), and fibroblast growth factor (FGF), Thus, a disruption of the autocrine/paracrine stimulatory loops based on these growth factors contributes to the efficacy of GH-RH antagonists as antitumor agents.
IGF-I and IGF-II are autocrine/paracrine growth factors with potent mitogenic effects on various cancers. IGF-I is also an endocrine growth factor, and elevated levels of serum IGF-I are considered an epidemiological risk factor for the development of prostate cancer, lung cancer, and colorectal cancer. The involvement of IGF-I (somatomedin-C) in breast cancer, prostate cancer, colon cancer, bone tumors and other malignancies is well established. Nevertheless, autocrine/paracrine control of proliferation by IGF-II is also a major factor in many tumors. IGF-I and IGF-II exert their proliferative and anti-apoptotic effects through the common IGF-I receptor. The receptors for IGF-I are present in primary human breast cancers, prostate cancers, lung cancers, colon cancers, brain tumors, pancreatic cancers, and in renal cell carcinomas. In several experimental cancers, such as those of the bone, lung, prostate, kidney, breast, ovary, intestine, pancreas, and brain, treatment with GH-RH antagonists produces a reduction in IGF-I and/or IGF-II levels, concomitant to inhibition of tumor growth (reviewed in Schally A V and Varga J L, Trends Endocrinol Metab 10: 383-391, 1999; Schally A V et al, Frontiers Neuroendocrinol 22: 248-291, 2001; Schally A V and Comaru-Schally A M, in: Kufe D W, Pollock R E, Weichselbaum R R, Bast Jr. R C, Gansler T S, Holland J F, Frei III E, Eds. Cancer Medicine, 6th ed. Hamilton, Ontario: BC. Decker, Inc., 2003, p. 911-926). In some cases, the expression of IGF-I receptors was also decreased by GH-RH antagonists. Thus the disruption of endocrine and autocrine/paracrine stimulatory loops dependent on IGF-I and IGF-II contributes to the antitumor effect of GH-RH antagonists.
In MXT breast cancer model, treatment with GH-RH antagonists inhibited tumor growth, reduced the mRNA level for GH and the concentration of GH peptide in tumors, and inhibited the mRNA expression for GH receptors (Szepeshazi K et al, Endocrinology 142: 4371-4378, 2001). GH was shown to act as a growth factor for MXT murine mammary carcinoma cells, MCF-7 human breast cancer cells and other tumor cell lines. Thus the inhibitory activity of GH-RH antagonists on local and serum GH levels contributes to their antitumor effect.
GH-RH antagonists have been shown to inhibit the mRNA levels and protein concentrations of VEGF in human androgen-sensitive and androgen-independent prostate cancer models (Letsch M et al, Proc Natl Acad Sci USA 100: 1250-1255, 2003; Plonowski A et al, Prostate 52: 173-182, 2002) and this phenomenon contributes to their antitumor effect, since VEGF plays an important stimulatory role in the neovascularization and growth of various tumors. Moreover, it was found that a GH-RH antagonist inhibited the VEGF secretion and proliferation of normal murine endothelial cells, apparently through a direct effect on these cells in vitro (Siejka A et al, Life Sci 72: 2473-2479, 2003).
Scientists have investigated various modifications of GH-RH to elucidate the relationship of the structure of GH-RH to its activity on the pituitary receptors, in an effort to provide synthetic congeners with improved agonistic or antagonistic properties. Thus, it was early established that GH-RH fragment comprising residues 1 to 29, or GH-RH(1-29), is the minimum sequence necessary for biological activity on the pituitary. This fragment retains 50% or more of the potency of native GH-RH. Subsequently, many synthetic analogs of GH-RH, based on the structure of hGH-RH(1-29)NH2 (SEQ ID NO: 1) peptide, were prepared. hGH-RH(1-29)NH2 (SEQ ID NO: 1) has the following amino acid sequence:
Tyr-Ala-Asp-Ala-lle5-Phe-Thr-Asn-Ser-Tyr10-Arg-Lys-Val-Leu-Gly15-Gln-Leu-Ser-Ala-Arg20-Lys-Leu-Leu-Gln-Asp25-Ile-Met-Ser-Arg29-NH2 (SEQ ID NO: 1)
A considerable number of patents and articles in the open literature disclose analogs of GH-RH which either act as agonists of GH-RH (i.e. act to stimulate the release of GH) or as antagonists of GH-RH (i.e. act to inhibit the release of GH) on the pituitary. Most of these peptides are derived from the GH-RH(1-29) peptide sequence, with specific structural modifications which account for their enhanced agonistic or antagonistic properties on the pituitary receptors. However, apart from a few exceptions, it is not known how these analogs would behave on cancer cells that express GH-RH receptors different from those found in the pituitary. Only a few published scientific studies tried to elucidate the structure-activity relationships and characterize the direct antagonistic (or agonistic) effects of GH-RH analogs on cancer cells and tumors (see Rekasi Z et al, Endocrinology 141: 2120-2128, 2000; Halmos G et al, Proc Natl Acad Sci USA 97: 10555-10560, 2000; Rekasi Z et al, Proc Natl Acad Sci USA 97: 10561-10566, 2000; Kiaris H et al, Proc Natl Acad Sci USA 99: 196-200, 2002), and no issued patents have dealt with this issue so far. Consequently, very little is known about the structural features in GH-RH analogs required for a direct antagonistic action on tumor cells.
The first described GH-RH antagonist, [Ac-Tyr1, D-Arg2]hGH-RH(1-29)NH2, which is generally termed as the “standard antagonist” in the literature, was found to prevent the activation of rat anterior pituitary adenylate cyclase by hGH-RH(1-29)NH2 (SEQ ID NO: 1). The same peptide blocked the action of GH-RH on its receptors in the pituitary and hypothalamus, and inhibited the pulsatile growth hormone secretion. The standard antagonist was also evaluated clinically (Ocampo-Lim B et al, J Clin Endocrinol Metab 81: 4396-4399, 1996; Jaffe C A et al, J Clin Endocrinol Metab 82: 634-637, 1997). Large doses of this antagonist (400 μg/kg) eliminated nocturnal GH secretion in normal subjects and inhibited the response to GH-RH. The standard GH-RH antagonist also reduced GH levels in a patient with acromegaly. However, for clinical use, much more potent antagonists of GH-RH are required.
The inventions mentioned below disclose GH-RH analogs with antagonistic or agonistic properties on the pituitary receptors for GH-RH. However it was not reported and not investigated whether these analogs could exert direct effects on tumor cells. U.S. Pat. No. 4,659,693 discloses GH-RH antagonistic analogs which contain certain N,N′-dialkyl-omega-guanidino alpha-amino acyl residues in position 2 of the GH-RH(1-29) sequence. Published application WO 91/16923 reviews earlier attempts to alter the secondary structure of hGH-RH by modifying its amino acid sequence. These earlier attempts include: replacing Tyr1, Ala2, Asp3 or Asn8 with their D-isomers; replacing Asn8 with L- or D-Ser, D-Arg, Asn, Thr, Gln or D-Lys; replacing Ser9 with Ala to enhance amphiphilicity of the region; and replacing Gly15 with Ala or Aib. When R2 in the analogs is D-Arg, and R8, R9, and R15 are substituted as indicated above, antagonistic activity is said to result. These antagonistic peptides are said to be suitable for administration as pharmaceutical compositions to treat conditions associated with excessive levels of GH, e.g., acromegaly.
The antagonistic activity of the hGH-RH analogue “[Ser9-psi[CH2—NH]-Tyr10]hGH-RH(1-29)” of U.S. Pat. No. 5,084,555 was said to result from the pseudopeptide bond (i.e., a peptide bond reduced to a [CH2—NH] linkage) between the R9 and R10 residues. However, the antagonistic properties of [Ser9-psi[CH2—NH]-Tyr10]hGH-RH(1-29) were said to be inferior to the standard antagonist, [Ac-Tyr1, D-Arg2]hGH-RH(1-29)-NH2. U.S. Pat. No. 5,550,212, U.S. Pat. No. 5,942,489, and U.S. Pat. No. 6,057,422, disclose analogs of hGH-RH(1-29)NH2 (SEQ ID NO: 1) said to have enhanced antagonistic properties and prolonged duration of action regarding the inhibition of GH-RH-evoked GH release. These properties are believed to result from replacement of various amino acids and acylation with aromatic or nonpolar acids at the N-terminus of GH-RH(1-29)NH2. The tumor inhibitory properties of antagonists featured in U.S. Pat. No. 5,942,489 and U.S. Pat. No. 6,057,422 have been demonstrated by using nude mice bearing xenografts of experimental human cancer models. It is noted that in U.S. Pat. No. 5,550,212, and in U.S. Pat. No. 5,942,489, R9 is always Ser, while R11 and R20 can be either Arg, D-Arg, or Cit. In the case of U.S. Pat. No. 6,057,422, R9 can be either Arg, Har, Lys, Orn, D-Arg, D-Har, D-Lys, D-Orn, Cit, Nle, Tyr(Me), Ser, Ala, or Aib, while R11 and R20 are always Arg.